A fuel cell is an electrochemical device that produces electricity from hydrogen or hydrogen rich gases without combustion, with water and heat as the only by-products. Fuel cell systems have several advantages over conventional power generation systems, including lower emissions, higher fuel efficiency, quieter operation, lower vibration, more reliable, lower maintenance and potentially lower capital costs. For instance, many phosphoric acid (PA) fuel cells installed at hospitals, banks, military bases, offices, factories and computing facilities have now accumulated many years of operation with only scheduled maintenance. These advantages enable fuel cells to offer cleaner and more efficient alternatives to existing power sources. Fuel cells are being developed for portable, residential, commercial, industrial, transportation and other power generations. They are vastly different from other power generation systems.
Individual fuel cells typically are stacked with bipolar separator plates separating the anode electrode of one fuel cell from the cathode electrode of an adjacent fuel cell to produce fuel cell stacks. These fuel cell stacks make the fuel cells operate at high efficiency, regardless of size and load. Distributed power generation from fuel cells reduces the capital investment and further improves the overall conversion efficiency of fuel to end use electricity by reducing transmission losses. Substantial advancements have been made during the past several years in fuel cells. Increased interest in the commercialization of polymer electrolyte membrane (PEM) fuel cells, in particular, has resulted from recent advances in fuel cell technology, such as more economical bipolar separator plates, higher current densities and the 100-fold reduction in the platinum content of the electrodes.
The electrolyte in PEM fuel cells is a solid ion conduction polymer, in which protons are mobile. That is why the PEM fuel cells are also called “proton exchange membrane fuel cells”. Ideally, PEM fuel cells operate with hydrogen. In the absence of a viable hydrogen storage option or a near-term hydrogen-refueling infrastructure, it is necessary to convert available fuels, typically CnHm and CnHmOp, collectively referred to herein as carbonaceous fuels, with a fuel process and system into hydrogen rich gases suitable for use in fuel cells. The choice of fuel for fuel cell systems will be determined by the nature of the application and the fuel available at the point of use. In transportation applications, it may be gasoline, diesel, methanol or ethanol. In stationary systems, it is likely to be natural gas or liquefied petroleum gas. In certain niche markets, the fuel could be ethanol, propane, butane or even biomass-derived materials. In all cases, reforming of the fuel is necessary to produce a hydrogen rich gas. However, the reforming catalyst (often Ni based) are poisoned by sulfur impurities in the carbonaceous fuels at temperatures less than 800° C. and therefore a hydrodesulfurization step or sulfur adsorption bed must be added to the fuel process and system prior to the reforming step. This is due to the adsorption of sulfur on the active metal catalyst sites. Sulfur also tends to increase coking rates which leads to further degradation of the reforming catalysts and unacceptable catalyst performance.
Regardless of the type-of reformer, the initial reforming product invariably contains CO. The bulk of the CO can be converted to additional hydrogen via the water gas shift (WGS) reaction. Hydrogen formation is enhanced by low temperatures, but is unaffected by pressure. Shift reactors can lower the CO level to about 0.5 to 2 mol %.
In this invention, an autothermal hydrodesulfurizing reforming catalyst (AHR catalyst) preferably as described in U.S. patent application Ser. No. 09/860,850 filed May 18, 2001 entitled “Autothermal Hydrodesulfurizing Reforming Catalyst”, is used for the autothermal hydrodesulfurizing reforming of sulfur bearing carbonaceous fuels into hydrogen rich gases, and a WGS catalyst, preferably a precious metal, sulfur tolerant, non-pyrophoric Pt/doped ceria/γ-alumina catalyst is used for WGS. Both catalysts' performances are not poisoned or degraded by sulfur impurities in the fuels. Sulfur impurities react in the autothermal hydrodesulfurizing reformer (AHR) and are converted to hydrogen sulfide, hydrogen and carbon oxides.
H2S and CO act as severe fuel cell electrocatalyst poison, while CO2 in the hydrogen rich gas acts as a diluent which increases the mass transfer resistance to decrease the fuel cell efficiency. In addition, CO2 produces CO via the reverse WGS reaction at the fuel cell anode. Therefore, an acid gas (CO2 and H2S)-selective removal and a CO clean-up system is usually required right ahead of the PEM fuel cells. Furthermore, a total elimination of CO2 is necessary for alkaline fuel cells because CO2 in the reformate reacts with the alkaline electrolyte and greatly hinders the performance of alkaline fuel cells.
As for the PA fuel cells, the phosphoric acid is tolerant to CO2 in the fuel and oxidant, unlike the alkaline fuel cells. Also, since the PA fuel cells operate at higher temperatures (about 180 to 230° C.) than PEM fuel cells and alkaline fuel cells, CO levels of 1 to 1.5% are acceptable and the final clean up step for CO via the methanator or the acid gas-selective WGS membrane reactor (AWMR) is no longer required. However, eliminating CO2 from a reformate increases into H2 concentration, greatly benefiting PA fuel cell power density, thus reducing cost.
It is, therefore, desirable to provide an improved sulfur tolerant dynamic fuel process and system with inline acid gas-selective removal for converting sulfur bearing carbonaceous fuels into hydrogen rich gases suitable for fuel cells and chemical processing applications.